Powerful Outflows and Feedback from Active Galactic Nuclei

AA53CH04-King ARI 27 July 2015 7:13 Powerful Outflows and Feedback from Active Galactic Nuclei Andrew King and Ken Pounds Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, United Kingdom; email: [email protected], [email protected] Annu. Rev. Astron. Astrophys. 2015. 53:115–54 Keywords First published online as a Review in Advance on supermassive black holes, accretion, M−σ relation, X-ray winds, molecular April 10, 2015 outflows, quenching of star formation The Annual Review of Astronomy and Astrophysics is online at astro.annualreviews.org Abstract This article’s doi: Active galactic nuclei (AGNs) represent the growth phases of the supermas- 10.1146/annurev-astro-082214-122316 Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2015.53:115-154. Downloaded from www.annualreviews.org sive black holes in the center of almost every galaxy. Powerful, highly ionized Copyright c 2015 by Annual Reviews. winds, with velocities ∼0.1–0.2c, are a common feature in X-ray spectra of All rights reserved luminous AGNs, offering a plausible physical origin for the well-known connections between the hole and properties of its host. Observability con- straints suggest that the winds must be episodic and detectable only for a few percent of their lifetimes. The most powerful wind feedback, establishing the M−σ relation, is probably not directly observable at all. The M−σ relation signals a global change in the nature of AGN feedback. At black hole masses below M−σ , feedback is confined to the immediate vicinity of the hole. At the M−σ mass, it becomes much more energetic and widespread and can drive away much of the bulge gas as a fast molecular outflow. 115 AA53CH04-King ARI 27 July 2015 7:13 1. INTRODUCTION 1.1. Supermassive Black Hole Scaling Relations Astronomers now generally agree that the center of almost every galaxy but the smallest contains a supermassive black hole (SMBH). In recent years it has become clear that the mass M of the hole correlates strongly with physical properties of the host galaxy. In particular the hole mass M appears always to be a fairly constant fraction of the stellar bulge mass M b, i.e., −3 M ∼ 10 M b (1) (Haring¨ & Rix 2004). Even more remarkably, observations give a tight relation of the form × 8 σ α M 3 10 M 200 (2) −1 between the SMBH mass and the velocity dispersion σ = 200σ200 km s of the host galaxy’s central bulge, with α 4.4 ± 0.3 (Ferrarese & Merritt 2000, Gebhardt et al. 2000; see Kormendy & Ho 2013 for a recent review). For many practical cases the relation is more conveniently written as × 7 σ α , M 2 10 M 100 (3) −1 with now σ = 100σ100 km s . Because observationally determining the SMBH mass generally involves resolving its sphere of influence, of radius GM M M R 8 8 pc 3 7 pc, (4) inf σ 2 σ 2 σ 2 200 100 8 7 with M = 10 M 8 M = 10 M 7 M, Equation 2 may represent a maximum SMBH mass for a given velocity dispersion σ (Batcheldor 2010). 1.2. Binding Energies At first glance, Equations 1 and 2 appear surprising. Equation 4 shows that the black hole’s gravity has a completely negligible effect on its host galaxy, which in most ways must be quite unaware of its existence. But we know (Soltan 1982) that the hole grew largely through luminous accretion of gas. This released energy, 2 61 EBH ηMc ∼ 2 × 10 M 8 erg (5) (where η 0.1 is the accretion efficiency), is far larger than the binding energy Access provided by California Institute of Technology on 01/11/17. For personal use only. 2 58 2 Annu. Rev. Astron. Astrophys. 2015.53:115-154. Downloaded from www.annualreviews.org ∼ σ ∼ × σ Ebulge M b 8 10 M 8 200 erg (6) 3 of a host bulge of stellar mass M b ∼ 10 M . The vast difference in these two numbers suggests that the host must notice the presence of the hole through its energy output, even though it is utterly insignificant in all other ways. We can already see how the black hole mass could correlate with galaxy properties—the hole grows by accreting gas, but in doing this it communicates some of its huge binding energy EBH back to the gas reservoir and so potentially limits its own growth. This suggests that the most relevant quantity to compare with EBH is not Ebulge but instead the gravitational binding energy of the bulge gas alone, i.e., Egas = fg Ebulge, (7) 116 King · Pounds AA53CH04-King ARI 27 July 2015 7:13 where fg < 1 is the gas fraction. In the following, we take this as fg ∼ 0.16, the cosmological mean value, giving a typical relation 3 EBH ∼ 2 × 10 Egas (8) −σ ∼ σ 4 / for a black hole close to the M relation (the right-hand side has an implicit factor 200 M 8). This picture requires the black hole to communicate some of its accretion energy to its host. But this process cannot be very efficient, as otherwise the hole could disrupt the host entirely or at the very least remove a large fraction of its gas. In this sense, the galaxy bulge leads a precarious existence. For much of its life it can ignore the threat that the SMBH poses, but we shall see that in the end this is always decisive if accretion continues. 1.3. Communicating the Energy: Feedback There are two main ways that the SMBH binding energy can potentially affect its surroundings. By far the stronger one (in principle) is through direct radiation. After all, this is how all the accretion energy is initially released. But we know from observation that most light escapes relatively freely from active galactic nuclei (AGNs). This suggests that radiation is in general not the main way the SMBH affects its host, and we discuss in detail why this is so in Section 7.4. The second form of coupling SMBH binding energy to a host bulge is mechanical. The huge SMBH accretion lumi- nosity drives powerful gas flows into the host, making collisions and communication inevitable. One form of flow often mentioned is jets—highly collimated flows driven from the immediate vicinity of the SMBH (see Fabian 2012 for a review). To turn these into a means of affecting most of the bulge requires a way of making the interaction relatively isotropic, perhaps with changes of the jet direction over time. Here we mainly consider another form of mechanical communication that automatically has this property already. This is the observed presence in many AGNs of near-isotropic winds carrying large momentum fluxes. 1.4. Powerful Ionized Winds Early X-ray observations of AGNs yielded soft X-ray spectra frequently showing the imprint of absorption from ionized gas, the warm absorber (WA) (Halpern 1984, Reynolds & Fabian 1995). More recent observations have found at least 50% of radio-quiet AGNs showing WAs in their soft X-ray (∼0.3–2 keV) spectra. The limited spectral resolution of the Einstein Observatory and Advanced Satellite for Cosmology and Astrophysics (ASCA) observations prevented important parameters of the WAs, in particular the outflow velocity and mass rate, to be determined with useful precision. The higher resolution and high throughput afforded by contemporary X-ray observatories Chandra, XMM-Newton,andSuzaku has transformed that situation over the past Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2015.53:115-154. Downloaded from www.annualreviews.org decade, with the WA being shown, typically, to be dominated by K-shell ions of the lighter metals (C, N, O, etc.) and Fe-L, with outflow velocities of several hundred kilometers per second (Blustin et al. 2005, McKernan et al. 2007). A more dramatic discovery made possible with the new observing capabilities was the detection of blueshifted X-ray absorption lines in the Fe K band, indicating the presence of highly ionized outflows with velocities v ∼ 0.1−0.25c (Chartas et al. 2002; Pounds et al. 2003; Reeves et al. 2003). In addition to adding an important dimension to AGN accretion studies, the mechanical power of such winds, which for a radial flow depends on v3, was quickly recognized to have a wider potential importance in galaxy feedback. Additional detections of high-velocity AGN winds were delayed by the weakness of absorption in such highly ionized gas combined with strongly blueshifted lines in low-redshift objects www.annualreviews.org • AGN Outflows and Feedback 117 AA53CH04-King ARI 27 July 2015 7:13 coinciding with falling telescope sensitivity above ∼7 keV. However, further extended observations, particularly with XMM-Newton, found evidence in five additional AGNs for outflow velocities of ∼0.1–0.2c (Cappi 2006). Some doubts remained as to how common high-velocity outflows were, as the majority of detections were of a single absorption line (with consequent uncertainty of identification) that had moderate statistical significance, raising concerns of publication bias (Vaughan & Uttley 2008). In addition, only for PG1211+143 had a wide-angle outflow been directly measured, confirming a high outflow rate of mass rate and mechanical energy in that case (Pounds & Reeves 2007, 2009). These residual doubts were finally removed following a blind search of extended AGN observations in the XMM-Newton archive (Tombesi et al. 2010), finding compelling evidence in 13 (of 42) radio-quiet objects for blueshifted Fe K absorption lines with implied outflow velocities of ∼0.03–0.3c.

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